A low-density barium photoplasma has been generated by two-step photoionization in an atomic beam. The transition from independent particle nature to collective effects has been observed in a V-shaped potential well. The potential is produced by a plate-grid-plate geometry in which the grid is negatively biased while the plates are grounded. Analytical expressions for the ion currents on the plates and grid have been deduced for a charge particle moving in the potential well and compared with experiments. The results are in good agreement and gave a method to estimate the ion transmission factor for the grid.
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